1. Field of the Invention
The present invention relates generally to automatic transmissions for vehicles and, more particularly, to a four-speed automatic transmission for an automotive vehicle.
2. Description of the Related Art
A conventional automatic transmission includes a hydrodynamic torque converter to transfer engine torque from an engine crankshaft to a rotatable input member of the transmission through fluid-flow forces. The transmission also includes frictional units, typically referred to as clutches, which couple the rotatable input member to one or more members of a planetary gearset. Other frictional units, typically referred to as brakes, hold one or more members of the planetary gearset stationary during the flow of power. Such transmissions also typically provide for one or more planetary gearsets in order to provide various ratios of torque and to ensure that the available torque and the respective tractive power demand are matched to each other.
An example of an automatic transmission having a plurality of planetary gearsets is a four-speed automatic transmission disclosed in U.S. Pat. No. 4,875,391 to Leising et al. This patented four-speed automatic transmission includes a torque converter with a lock-up clutch, three drive clutch assemblies, two brake assemblies, and two planetary gearsets.
A disadvantage of the above patented four-speed automatic transmission is that to provide a 0.75 overdrive ratio in the arrangement of the planetary gearsets results in an excessive ratio step between first and second gear, e.g., 3.00 and 1.50, respectively.